A federal judge has blocked the U.S. Department of Justice from carrying out an effort to obtain state voter-registration rolls and compare them against a federal immigration database, ruling that the initiative was unlawful. The decision marks a significant setback for a nationwide voting-enforcement strategy that states and voting-rights groups argued could lead to erroneous voter removals and place improper pressure on election officials to purge their rolls.

At the center of the dispute was the federal government’s attempt to use immigration data as a screening tool for voter eligibility.